List comprehension and generator expressions - Intermediate Python Programming p.4

Key Takeaways

what's going on everybody Welcome to part four of our intermediate Python Programming tutorial Series in this tutorial we're going to be talking about list comprehension in sort of generators but really generator expressions and we'll be talking about generators more uh later on so first of all to start uh one of the most common generators that you've probably been using is range So when you say for I in range of five this does not generate a list 0 1 2 3 four it's a generator and it it's it generates a stream in the range of 0 to 5 which is produces 0 1 2 3 4 so but it doesn't do that and save it all into memory as opposed to say list comprehension or a list that actually obviously would would store that into memory so in Python 2 um Range actually isn't a generator and it does store that entire list into memory which is why if you in Python 3 you can get away with saying like I equals range this that's okay you're G to be just fine when you do that right we can do that real quick okay boom it's already done all right but in Python 2 if you run that code it's going to be like like it's just not going to happen and you're going to have to close out of python okay so because it's generating a list and it's just going to blow your memory so generator now fundamentally we'll I'll have to keep hitting this point as we go through this this entire course really but list comprehension or a list is going to be faster but it's going to use your memory right it's going to use your RAM to store that list and that's why it's faster it's already loaded boom it's in memory generators on the other hand are going to be slower but they're not going to use as much memory and slower is like with an asterisk because building a list actually like when it comes time to to build and save that list to a very able that can actually sometimes be a a pretty time intensive process so actually sometimes generators are faster but in most like raw cases the generator is going to be slower it's just not going to blow out your memory cool now that we have that out of the way how do we build these things so maybe you've seen things like I4 I in range five and maybe it's uh XYZ equals I for I in range five what the heck is that so what that that is is basically it's the same as saying XYZ equals empty list for I in R not the same but it produces the same output for someone corrects me on that one uh for I in range five XY z. append I okay these two things do the exact same thing we can print XYZ and then do the same thing again okay same output so that's list comprehension now what's a what about a generator expression so a generator Expressions literally the exact thing just instead we use parenthesis generator boom okay but those parentheses make all the difference because this is not going to store this into memory it make it's a generator now so uh so for example if we do print um let's do it up here XYZ XYZ run one of them's an actual list it's in our memory the other one H weird it's a generator object and what do we do with a generator object well as you might guess from before when we did I for I in range five you iterate over a generator object so in this case you would say 4 I in XYZ print I and then you're able to iterate over it but the magical thing about a generator is it was not it didn't store that range um or it didn't store this right as a list 0 1 2 3 4 as opposed to this one so if this range instead was this don't try this at home I'll probably regret this let's see what are we using us here 500,000 let's do that we'll actually do that that'll help us sort of visually time it I thought it would be done by now but maybe not my goal is to show you that as soon as pretty much list comprehension is done the other range would be done surely range of 5,000 is not too much to ask it's too much to ask everybody I'm so sorry oh my gosh we're still oh the problem is we're printing okay yeah no that's the problem sorry dumb of course let's open that back up again I'm pretty sure that's the problem let's just say done try again okay yeah so rather than uh this now let's add let's go back to 500,000 or what is this five million that's five million that's okay we'll do five million yeah okay 5 million is acceptable 5 billion okay that one's taking a little longer took off one Zer we'll see if that's any fast there we go so okay that's a good one so waiting done and then the generator is like bam right because we didn't need to store it into memory okay so that's why the generator was much quicker to generate but then as you start to actually want to iterate through it or do something with it the generator is going to take um take a longer time okay so that's an introduction to list comprehension and generators in the next tutorial we are going to apply a function to one of these and also talk about kind of embedding them into themselves and stuff so if you have any questions or comments concerns whatever feel free to leave them below otherwise as always thanks for watching thanks for all the support subscriptions and until next time